Timing belt replacement, I lost timing marks

Experienced mechanic messes up (me) removed old timing belt I saw marks aligned but old belt erased marks. I'm trying to install the new one starting from the crankshaft up to the cam #2 align mark to go to cam #1 align mark but the belt is not tight tensioner go up all the way belt still not tight proceeding from cam#2 to cam #1 after installing on crankshaft sprocket is one tooth after the mark. I got lost here. Can you please help me? What is the best way to make sure marks are correct as I remove the spark plug to check if the piston is up as I said before marks were all aligning but old belt has no marks.